1. Field of the Invention
The present invention relates to a solid-state image sensor and, more particularly, to a technique which may effectively be used for, for example, image input devices for recording still images such as the images of letters and figures.
2. Description of the Related Art
One type of solid-state image sensor which consists of a combination of photodiodes and switching MOSFETs has heretofore been known. This type of solid-state image sensor is disclosed, for example, in Japanese Patent Laid-Open No. 56- 152382(152382 /1981). Solid-state image sensors of the type described above are used in television cameras for monitoring or those for domestic use. In such a television camera, an optical lens is provided with an automatic diaphragm mechanism.
The above-described lens that is provided with an automatic diaphragm mechanism needs relatively complicated mechanical parts, and this leads to an increase in size of the lens unit in a television camera and a rise in the production cost and also involves inferior response. For this reason, devices that utilize such optical lenses, e.g., image input devices for recording letters or figures, suffer from the problem that the digital pattern processing as a pre-processing for recognizing such still images is complicated. More specifically, in the case of continuously inputting images of objects such as a document consisting of a plurality of letters or figures which are drawn on different plurality of kinds of paper or the like having different levels of lightness, there may be variations in level of picture signals obtained in accordance with the level of the lightness of the paper. Further, since the above-described automatic diaphragm mechanism is composed of relatively complicated mechanical parts, the mechanical portions may become worn, and the mechanism is therefore unsatisfactory in terms of reliability.